Fair Value Measurements (Details) (USD $)																													3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Convertible Debt [Member]	Dec. 31, 2011 Convertible Debt [Member]	Dec. 31, 2010 Convertible Debt [Member]	Mar. 31, 2012 6.625% Senior Notes [Member]	Dec. 31, 2011 6.625% Senior Notes [Member]	Mar. 31, 2012 6.50% Senior Notes [Member]	Dec. 31, 2011 6.50% Senior Notes [Member]	Dec. 31, 2010 6.625% Senior Notes due 2019	Mar. 31, 2012 Fair Value, Measurements, Recurring [Member] Level 1	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 1		Mar. 31, 2012 Fair Value, Measurements, Recurring [Member] Level 1 Derivatives not designated as hedging instruments	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 1 Derivatives not designated as hedging instruments		Mar. 31, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]		Mar. 31, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member] Derivatives not designated as hedging instruments	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member] Derivatives not designated as hedging instruments		Mar. 31, 2012 Fair Value, Measurements, Recurring [Member] Level 3	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 3		Mar. 31, 2012 Fair Value, Measurements, Recurring [Member] Level 3 Derivatives not designated as hedging instruments	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Level 3 Derivatives not designated as hedging instruments		Mar. 31, 2012 Net Profits Plan [Member]	Mar. 31, 2011 Net Profits Plan [Member]	Mar. 31, 2012 Net Profit Plan liability [Member]	Mar. 31, 2011 Net Profit Plan liability [Member]
Financial assets and liabilities measured at fair value on a recurring basis
Discount rate to calculate liability pools currently in payout (as a percent)																															12.00%
Cash payments made under the Net Profits Plan related to divested properties																													$ 286,000	$ 4,300,000
Amount paid for settlement of liabilities																															0	2,600,000
Assets:
Derivatives														0	0	[1]				98,052,000	86,875,000	[1]				0	0	[1]
Proved Oil and Gas Property, Successful Effort Method	4,657,347,000	4,378,987,000										0	[2]					0	[2]					139,992,000	[2]
Unproved Oil and Gas Property, Successful Effort Method	130,688,000	120,966,000										0	[2]					0	[2]					15,809,000	[2]
Liabilities [Abstract]
Derivatives														0	0	[1]				76,161,000	55,681,000	[1]				0	0	[1]
Net Profits Plan Liability											0	0	[1]				0	0	[1]				111,670,000	107,731,000	[1]
Interest rate on notes (as a percent)			3.50%	3.50%		6.625%	6.625%	6.50%	6.50%	6.625%
Approximate estimated fair value of notes			$ 374,000,000		$ 394,000,000	$ 371,000,000	$ 359,000,000	$ 373,000,000	$ 360,000,000

[1] This represents a financial asset or liability that is measured at fair value on a recurring basis.

[2] This represents a non-financial asset or liability that is measured at fair value on a nonrecurring basis.